Solar screening film for a vehicle windshield

ABSTRACT

A solar screening film for a motor vehicle windshield comprises an interference filter having one or more heat reflecting metal layers and one or more coacting reflection-suppressing layers to minimize reflection and enhance visible transmission and a gradient band continuation of the interference filter above the direct line of forward vision through a windshield when the film is disposed therein in a motor vehicle, for reflective rejection of infra-red solar energy, such band including a light-reflecting metal layer gradually increasing in thickness toward an edge of the film.

BACKGROUND OF THE INVENTION

This invention relates to reducing heat buildup in a motor vehicle andmore particularly to doing so by the structure of the windshield.

Windshields of vehicles such as automobiles are conventionally oflaminated glass comprising thermoplastic interlayer, such as plasticizedpolyvinyl butyral or polyurethane sheeting, sandwiched between twolayers of glass. These assemblies are mounted in openings across theupper front ends of the vehicle bodies and, in modern designs, areinclined backwardly at a substantial angle from the vertical therebyadmitting sunlight into large areas of the vehicle interior. Theinterlayer conventionally has an integral marginal band oflight-absorbing dye above the driver's direct line of vision through thewindshield to reduce sunlight glare. This band is graduated inintensity, being greatest near the upper periphery of the interlayerwhen in place in a windshield and gradually diminishing to an almostimperceptible level at the lower edge of the band.

Solar screening films have been incorporated into these assemblies toreduce solar radiation influx while maintaining high luminous or visiblelight transmission, thus reducing temperature buildup within the vehiclefrom exposure to the sun's rays without adversely affecting visibilitythrough the windshield. Such films are called interference or inducedtransmission filters and comprise a multi-layer coating of at least onelayer of reflective metal sandwiched between reflection-suppressingdielectric layers. Representative structures for motor vehiclewindshields are disclosed in International Publication No. WO88/01230and U.S. Pat. No. 4,799,745.

For safety purposes windshields containing solar screening films muststill comply with a federally mandated 70% minimum luminous transmissionlevel in the area of the driver's direct field of view, and forconvenience should desirably still have the capability of minimizingglare. Moreover, there is a recent trend toward extending windshieldsback into the roof panel to provide a more open, spacious feeling to theoccupants. The level of solar flux against an overhead windshieldextension in the roof area, which is orthogonal to the sun's rays, isessentially maximum. However, absorptive solar rejection by a dyed bandor by heat absorbing glass has little effect on control of the internalequilibrium temperature of a vehicle parked in direct sunlight, becauseabsorbed heat contributes substantially to increasing the interiorvehicle temperature. Furthermore, though reducing luminous transmissionin the visible region, a dyed gradient band provides no attenuation inthe near infra-red region (700-3000 nm).

SUMMARY OF THE INVENTION

Now improvements have been made in motor vehicle windshields containingsolar energy control features which overcome or mitigate prior artshortcomings.

Accordingly, a principal object of this invention is to provide animproved solar screening film for a motor vehicle windshield.

An additional object is to provide a solar screening film for a motorvehicle windshield which has an improved form of the anti-glare gradientband usually found in modern windshields.

Another object is to provide such a film which is capable of increasedsuppression of equilibrium temperatures reached within the vehicleduring solar soaking, for example during parking lot exposure, whilesimultaneously exhibiting good transmission performance in the visiblespectra range.

A further object is to provide such a film with a gradient band capableof reflective rather than absorptive solar rejection.

Other objects of this invention will in part be obvious and will in partappear from the following description and claims.

These and other objects are accomplished by providing a solar screeningfilm for a motor vehicle windshield comprising an interference filterwhich includes one or more light-reflecting metal layers and one or morereflection-suppressing layers for coacting with the one or more metallayers to minimize reflection and enhance visible transmission, and agradient band continuation of such filter above the direct line ofvision through a windshield when the film is disposed therein, forreflective rejection of solar energy, such band including alight-reflecting metal layer gradually increasing in thickness toward anedge of the film.

The thickness of the metal layer of the gradient band continuationpreferably increases to about 200 to 1000 Angstroms (A) from about 60 to200 A at the confluence with the interference filter segment. Thegradient band continuation preferably includes extensions of thereflection-suppressing layers of substantially the same thickness as inthe interference filter. When a windshield extension is intended, themetal layer continuation extends preferably into the roof line of thevehicle at a thickness greater than the minimum thickness of the metallayer extension, for example at essentially the same thickness as thatof the maximum thickness of the wedge-shaped profile of the gradientband thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

In describing the invention, reference will be made to the accompanyingdrawings wherein:

FIG. 1 is a partial, cross sectional view of a solar screening filmaccording to the invention;

FIG. 2 is a central, vertical, sectional view of a vehicle windshieldand windshield extension containing the solar screening film of FIG. 1highlighted in enlarged form within a circumscribing circle;

FIGS. 3 and 6 are schematic illustrations of the thickness profile ofsolar screening films of the invention; and

FIGS. 4, 5, 7 and 8 are normal incidence plots graphically illustratingmeasured transmission and reflection spectra in specific areas oflaminates containing solar screening films of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, solar screening film 10 in FIG. 1 formotor vehicle windshield W (FIG. 2) comprises a high-transmissioninterference filter segment 12 in the form of a multi-layer coatingwhich, in the embodiment shown, includes two light-reflecting silvermetal layers 14, 16 of substantially uniform thickness separated bydielectric spacer layer 20, with two opposing planar surfaces of each of14, 16 being in face-to-face contiguous engagement withreflection-suppressing dielectric layers 18, 21.

Solar screening film 10 further comprises gradient band continuation 22of high-transmission interference filter 12 above the direct line offorward vision through windshield W when solar screening film 10 isdisposed therein in a motor vehicle, for reflective rejection of solarenergy. Gradient band continuation 22 includes extension 24 which ispreferably integral with light-reflecting metal layer 14 and iswedge-shaped in thickness profile insofar as gradually upwardlyincreasing (though not necessarily linearly) toward a marginal edge ofsolar screening film 10. Extension 24 need not be integral with a metallayer of the high-transmission interference filter segment 12, butalternatively could be deposited as a separate gradient layer, forexample on the outboard surface of layer 38, which is further identifiedbelow.

The direct line of forward vision through windshield W is the highvisibility area subjacent imaginary line 26 in FIG. 2. The location ofline 26 in windshields of U.S. vehicles is usually defined by the label"AS1" imprinted on one of the glass layers. The wedge-shaped integralgradient band continuation 22 is schematically highlighted in FIG. 2 bythe gradual increase in multiple dot shading intensity from confluence26 with interference filter segment 12 to marginal edge 28. Continuation22 of solar screening film 10 when in windshield W encompasses zone 27between lines 26, 28 which is about the upper 1/3 of the windshield andis above the direct line of forward vision through windshield W.

When optional windshield extension 30 (FIG. 2) is present, solarscreening film 10 preferably includes a high reflection region 29comprising a further continuation of metal layer 24 having a thicknessgreater than the minimum thickness, and exemplarily essentially equal tothe maximum thickness of wedge-shaped extension 24 of gradient band 22.Extension 30 encompasses zone 29 within imaginary lines 28, 32.

Gradient band continuation 22 includes extensions 34, 36 and 38 (FIG. 1)of each of reflection-suppressing dielectric layers 18, 21 and spacerlayer 20 of high-transmission interference filter 12, of substantiallyuniform thickness, which is substantially the same as the thickness oflayers 18, 20, 21. Continuation 22 further includes unthickened metallayer extension 37 of metal layer 16 of high-transmission segment 12.

Solar screening film 10 is supported on a transparent substrate which,in the FIG. 1 embodiment, is thermoplastic sheet 40 of biaxiallyoriented polyethylene terephthalate. Before incorporation intowindshield W, the composite laminate of solar screening film 10 ontransparent substrate 40, which is illustrated as 42 within the circlein FIG. 2, is preferably sandwiched between and laminated by heat andpressure to two layers of plasticized polyvinyl butyral 44 and 46 (FIG.2). In preparing windshield W, the encapsulate of sandwich components42, 44 46, is further laminated in conventional manner between outerfloat glass layers 48,50.

FIG. 3 schematically illustrates the metal layer thickness profile of atypical embodiment of solar screening film 10. The thicknesses of layers14, 16 in the high visibility region of windshield W below 26 containinginterference filter 12 are 110A. The thickness of metal layer 24 ofgradient band 22 gradually increases from about 60 to 200 A (e.g. 110Aas shown) at the confluence (26 in FIG. 2) with the upper edge ofinterference filter 12 up to about 200 to 1000 A (e.g. 450A as shown) atthe marginal edge (28 in FIG. 2) of film 10 in the absence of awindshield extension. Alternatively, without extension 30, layer 24 mayhave a vertically short constant thickness portion at the upper end ofthe tapered section. With extension 30 present, high reflectance region29 typically has essentially the same thickness (450A in FIG. 3) as thatof the maximum thickness of the wedge-shaped thickness profile ofgradient band 22.

FIG. 6, schematically illustrates an optional, anti-glare feature. Ananti-glare backing is positioned behind the integral gradient bandcontinuation for suppressing luminous reflection by absorption fromwithin the vehicle compartment when solar screening film 10 is disposedin windshield W. Such anti-glare backing comprises gradient metal layer52 aligned behind gradient band continuation 22 commencing (zerothickness) at the confluence 54 of interference filter 12 and gradientband continuation 22, and complementing the thickness profile ofgradient band continuation 22 insofar as gradually increasing to about200A opposite the maximum thickness end of band 22 and then, if awindshield extension is present, continuing parallel to high reflectanceregion 29 at a constant level of 200A. The relative thickness of layer52 need not be linear or proportional to that of section 22 and ischosen to balance glare suppression while retaining reasonabletransmission.

The interference filter component of the solar screening film of theinvention is of the Fabry-Perot type designed principally through theappropriate selection of materials and thicknesses to maximize i)transmission of visible or luminous (300-700 nm) and ii) reflection ofheat-generating infra-red (700-3000 nm) portions of the solar spectrum.Such filter consists of multiple, sequentially deposited successiveplanar layers of angstroms-thick metal and dielectric coatings arrangedin a predetermined sequence in face-adhering, contiguous contact witheach other, of the type generally disclosed in U.S. Pat. Nos. 3,682,528,4,179,181 and 4,799,754.

The interference filter component must contain one, preferably two ormore heat (near IR) reflecting metal layers which, when operativelypositioned in a windshield in the direct line of forward vision of anoccupant such as the driver, provide at least 70% visible lighttransmission of normal incidence measured as specified in ANSI Z26.1,which is the minimum allowed by the U.S. automotive industry. Preferablyvisible light reflectance, normal from the surface of the filter is lessthan 16%. The metal layers of the interference filter must be verticallyseparated in the thickness direction from each other by one or moredielectric layers conventionally used in interference filters whichcoact with the one or more metal layers so reflections of visible lightfrom various interfaces interfere destructively thereby enhancingvisible transmission.

The gradient band component reflectively rejects near infra-red solarenergy and is an integral extension of a light-reflective metal layer ofthe interference filter component of gradually increasing thicknesstoward an upper marginal edge of the solar screening film. Though morethan one metal layer may be present and desirable to enhance visibletransmission performance in the high-transmission interference filtersegment, it is generally unnecessary (though possible if desired) tothicken more than one of such layers in the gradient band continuation,since two or more gradient metal layers tend to produce a) more than onecolor across the gradient band and b) non-uniform color due to slightvariations in layer thicknesses. As metal layer thickness increasesacross the region defined by such gradient metal band component, solarand luminous reflection increases, luminous transmission decreases andsolar rejection increases.

Usable metals in the solar screening film comprise silver, aluminum,chrome, nickel, brass, gold, stainless steel, copper, palladium andalloys or claddings of any of the foregoing. Silver is preferred. Eachmetal layer should be continuous to maximize visible light transmissionand near infra-red solar rejection properties.

The metal layer(s) thickness in the high-transmission interferencefilter segment should be between 60 to 200, preferably 70 to 180 andmost preferably 80 to 160A. For maximum reflection attenuation by theinterference filter, when using plural (for example two) metal layers,reflection from each should be equal and this is best achieved by havingthe thickness and composition of each layer nearly identical.

Metal layer thickness of the gradient band continuation at theconfluence with the interference filter segment is the same as thethickness of the layer of which it is an extension, which, as presentedabove, is 60 to 200, preferably 70 to 180 and most preferably 80 to 160A. This thickness gradually, though not necessarily linearly, increasesto a maximum near the roof line of the vehicle which can vary widelydepending on desired performance, but is generally about 200 to 1,000,preferably about 300 to 600A. When a windshield extension (which can bethe entire vehicle roof) is present and it is desired to extend thesolar screening film into such area by including a high reflectanceregion, the metal layer thickness in such region is greater than that ofthe gradient band continuation at the confluence with the interferencefilter, but can otherwise vary widely. For convenience, the metal layerthickness in such high reflectance region is constant and substantiallyequal to the maximum thickness of the gradient band continuation which,as indicated, is generally about 200 to 1,000, preferably 300 to 600A.

The dielectric layer(s) of the solar screening film must be essentiallytransparent over the solar range (i.e. form 325 to 2125 nm) and at leastone must exist between a pair of metal layers when plural layers are inthe interference filter element. Exemplary usable dielectric materialsinclude SiO, Sio₂, Ta₂ O₅, WO₃ . . . SnO₂, Al₂ O₃, MgF₂, ZnS, and ZnO₂.TiO₂ is preferred for each dielectric layer.

As generally known, varying the thickness and composition of adielectric layer spaced between two reflecting metal layers, willconsiderably vary the optical transmittance/reflection properties of theinterference filter. More specifically, varying the thickness of thespacing dielectric layer varies the wave length associated with thereflection suppression (or transmission enhancement) band. Generally,the thickness of spacing dielectric layer(s) in the solar screening filmshould be between about 200 to about 1200 and preferably between 450 to1000 A to obtain the desired optical properties for a commerciallyacceptable product. Metal oxide dielectric layers less than about 200 ormore than about 1200 A result in very low luminous transmittance.

Exterior dielectric layers contacting metal layer surfaces opposite tometal surfaces contacting spacing dielectric layer(s) are preferablyused in the interference filter element to enhance anti-reflectionperformance. Exterior dielectric layers generally should have a higherrefractive index than glass i.e. greater than 1.5 and preferably greaterthan 1.8. The thickness of such exterior or outside dielectric layer(s)is generally less than the spacing dielectric layer(s) and should beabout 100 to about 600 and preferably 160 to 500A.

Reflection-suppressing dielectric layers of substantially uniformthickness are preferably present in the gradient band continuationsegment and optional high glare region of the solar screening film asextensions of and at substantially the same thickness of thereflection-suppressing layers of the interference filter. Their presencei) provides a suitable surface to which the deposited metal layer willadhere; ii) minimizes agglomeration of the metal layer depositedthereon; iii) provides chemical and mechanical protection for the metallayer and iv) maximizes visible transmission for a given solarrejection.

The solar screening film of the invention rests in supporting face toface contact on the surface of a transparent substrate which itself cancomprise one or plural layers. Such substrate can be selected from avariety of materials with choice generally governed by compatibilitybetween the solar screening film and the substrate. Usable substratesshould not be prone to stretch to avoid cracking the metal/dielectriclayers and should be free of excess volatiles such as plasticizers,water vapor or absorbed gases. The dielectric layer of the solarscreening film in direct contact with the substrate should adhere wellto the substrate surface. Generally the solar screening film of theinvention will adhere well to glass and certain flexible plastics suchas polyesters, cast acrylics, polycarbonates, chlorinated plastics andepoxies. Polyurethanes and plasticized polyvinyl butyral as a substratecomponent in direct supportive contact with the solar screening film aretoo soft and extensible. Preferred substrates are sheets of transparentmaterials such as glass or non-extensible flexible plastics such aslinear polyesters, e.g. molecularly oriented (i.e. biaxial or uniaxial)polyethylene terephthalate which is commercially available fromHoechst-Celanese Company as Hostaphan 4400-400. In a preferredconstruction the layers of the solar screening film are sequentiallydeposited on a flexible sheet substrate of biaxially orientedpolyethylene terephthalate (PET), and then the substrate carrying thesolar screening film is encapsulated within two layers of conventionalplasticized polyvinyl butyral (PVB), one layer abutting the PETsubstrate and the other abutting the top layer of the solar screeningfilm. The multi-layered encapsulate or sandwich containing PVB as theouter layers is then conventionally laminated between two rigid memberssuch as glass panes, or alternatively may be used as a bilayer structurewhere lamination is to only one such rigid member.

Individual layers of the solar screening film are deposited by knownvacuum coating techniques such as vacuum evaporation or sputtering.Usable methods include evaporation (resistance or laser heated, orelectron-beam vaporization) and sputtering (diode or magnetron) undernormal or reactive conditions. Magnetron sputtering is preferred. Alayer of graduated metal thickness may be deposited in any of severalways as follows:

(1) The speed of the substrate can be varied to change the duration ofdeposition on different regions of the substrate.

(2) The target area exposed to the substrate can be varied to providethe appropriate thickness profile such as by masking rectangular targetsor using one or more shaped targets.

(3) A single target might be segmented to allow a different voltage tobe applied to each segment. Those segments with higher applied voltageswould have correspondingly higher deposition rates.

(4) The rate enhancing magnetic fields can be varied to achieve fasterdeposition in selected areas of the target.

The invention is further described in the following examples which arefor illustration only and not intended to imply any limitation orrestriction on the invention.

Samples described in Examples were prepared on 5 cm square or 5×15 cm 90mm thick glass plate substrates. Prior to deposition of the solarscreening film thereon, each glass plate was mechanically washed indetergent solution then rinsed and immersed in a 50/50 volume % solutionof concentrated sulfuric acid and 30% hydrogen peroxide. After removalthe plates were rinsed in distilled water and isopropanol, blown drywith nitrogen and placed in a vacuum chamber of the sputter coatingapparatus.

All vapor deposited layers were sequentially applied one on top of theother by magnetron sputtering using a Leybold Heraeus Z400 sputtercoater, the first layer being deposited on the glass plate surface.Tungsten oxide was reactively prepared using round metallic targetswhich had a diameter of about 7.5 cm and an area of about 44 squarecentimeters. Layer thicknesses were monitored using an Inficon XTCcrystal monitor.

After the vacuum chamber was pumped to a base pressure of less than1×10⁻⁵ mbar, the appropriate gas mixture (see additional details below)was introduced for sputtering.

Typical conditions used to deposit the layers are in Table I. Thetungsten layer in parentheses was deposited only where interior glarereduction was sought (Example 3). The first 60 A of a top dielectriclayer was deposited under especially mild conditions (lower wattage andlower oxygen partial pressure) to prevent damage to the underlyingsilver layer. Deposition times were varied to achieve the thicknessesspecified. For a graduated metal layer thickness, the sputtering targetwas moved along the length of a 5×15 cm substrate at various rates asrequired to achieve the desired thickness profile.

                  TABLE I                                                         ______________________________________                                                  Air     O.sub.2               Dep.                                            Flow    Flow    Pressure                                                                              Power Rate                                  Layer     (sccm)  (sccm)  mBar    watts (A/sec)                               ______________________________________                                        tungsten oxide                                                                          30      6       3.5 × 10.sup.-5                                                                 32    3                                     silver    30      0       2.5 × 10.sup.-5                                                                 29    10                                    (tungsten)                                                                              30      0       2.5 × 10.sup.-5                                                                 116   10                                    tungsten oxide                                                                          30      1       3.0 × 10.sup.-5                                                                 19    1                                     tungsten oxide                                                                          30      6       3.5 × 10.sup.-5                                                                 32    3                                     ______________________________________                                    

Optical properties were determined with a Perkin Elmer 330 UV/VIS/NIRspectrophotometer. When reflectance measurements were made the glassplate containing the layered solar screening film was placed nearest theintegrating sphere. Laminates were prepared with the outer vapordeposited layer in face to face contact with a PVB interlayer in theform of 30 mil thick Saflex® sheet. The sequential layers of a laminatewere: coated glass substrate/PVB/glass. In preparing a laminate thecoated glass/PVB/glass assembly was preheated at 150° C. for 15 min. anda pressure of 40 psi applied for 5 min. while maintaining suchtemperature. With the pressure maintained the laminate was then allowedto cool to room temperature.

Abbreviations used in Examples are defined as follows:

Tv, Rv and Av=respectively, visible transmission, reflectance andabsorption,

Ts, Rs and As=respectively, solar transmission, reflectance andabsorption,

SR=solar energy rejection calculated under standard summer conditionsspecified in ASHRAE Handbook, 1985, chapter 27- i.e. %SR=100%-%Ts-0.27×%As.

W03=tungsten oxide.

EXAMPLE 1

An integral gradient band was introduced into a three layer interferencefilter as a continuation of the metal layer by gradually increasing thethickness of a silver layer in a tungsten oxide/silver/tungsten oxidemulti-layer stack. The thickness of the silver layer was increased inthe gradient metal band from about 110A to about 450A while bothtungsten oxide layers on either side of the silver were held constant atabout 400A. FIG. 3 schematically depicts the silver layer thicknessprofile versus potential position in a windshield while FIG. 2 showsregion 27 as the actual windshield location of the gradient metal band.

After laminating 30 mil (0.76 mm) thick Saflex® sheet to the uppertungsten oxide layer of the stack, the transmission (FIG. 4) andreflection (FIG. 5) spectra were taken in the high transmissioninterference filter segment and the zone of maximum thickness of thegradient band region (respectively labeled "IF" and "GB" in FIGS. 4 and5) and the optical characteristics measured providing the followingresults:

                  TABLE 2                                                         ______________________________________                                        Region       % Tv    % Rv    % As  % Rs  % SR                                 ______________________________________                                        interference filter                                                                        72      17      20    33    48                                   gradient metal band                                                                        12      78      18    74    87                                   ______________________________________                                    

As apparent from FIG. 4 and Table 2, in the 400-700 nm range, visiblelight transmission of the interference filter segment through which avehicle occupant would look when a full-size optical element of thisconstruction is installed in a windshield, is desirably greater than 70%while reflection (FIG. 5) is 33%. On the other hand, reflectiverejection in the gradient metal band region above an occupant's directline of vision when such a film is positioned in a windshield is quitehigh as represented by the 74% Rs value in Table 2.

COMPARATIVE EXAMPLE 1A

This compares optical performance of the gradient metal structure ofExample 1 with a dyed gradient band conventionally used as an integralplastic section of a plasticized polyvinyl butyral interlayer. Suchinterlayer when installed in a laminated glass windshield wouldconventionally provide a dyed color band (called "tinted" below) acrossthe top of the windshield above the driver's direct field of vision anda clear, subjacent section (called "clear" below) opposite the directfield of vision through the windshield. Such a structure is typicallyshown in FIG. 1 of U.S. Pat. No.3,038,206. The particular interlayer ofthis Comparative Example had a blue gradient band and is available fromMonsanto Company as Saflex® TG45. The measured optical properties ofsuch interlayer are in Table 3 following. Though not shown, the measuredtransmission spectra for the clear and tinted portions of the interlayerexactly tracked and were equal to each other in the near IR 700-3000 nmrange,

                  TABLE 3                                                         ______________________________________                                        Region  % Tv     % Rv     % As   % Rs   % SR                                  ______________________________________                                        Clear   88       8        14     7      17                                    tinted  12       5        46     6      40                                    ______________________________________                                    

Although the tinted section substantially reduces luminous transmissionin the visible (400-700 nm) region, it provides no attenuation in thenear IR region. Thus, though Tv is reduced to 12% in the tinted region,the overall solar rejection is only 40% in comparison with the 217%greater value of 87% SR in Example 1. The major solar rejectionmechanism is absorptive in the tinted band as evidenced by 46% As whichis not as effective as the major reflective rejection of 74% Rs achievedin Example 1.

EXAMPLE 2

To relate silver thickness to optical performance and explore functionalgradient metal band thicknesses, samples were prepared as in Example 1(i.e. three layer stacks of tungsten oxide/silver/tungsten oxide) eachwith the same thicknesses of tungsten oxide (450 A next to the glasssubstrate and 500A for the external top layer) but with silver layerthicknesses which varied between samples (though silver layer thicknessin each sample was uniform). Optical properties are in Table 4 whichshows solar rejection increasing as silver metal layer thicknessincreases.

                  TABLE 4                                                         ______________________________________                                              Silver                                                                        Thickness                                                               Sample                                                                              (A)        % Rs    % As  % Tv  % Rv  % SR                               ______________________________________                                        A57   120        23.3    21.0  78.6  10.1  38.6                               A56   140        28.7    22.0  75.2  13.5  44.8                               A63   160        31.3    24.7  69.4  15.7  49.3                               A58   200        40.0    22.4  62.8  23.5  56.4                               A62   300        54.8    22.7  40.8  43.5  71.4                               A59   400        67.4    19.6  24.4  63.8  81.7                               A64   500        73.1    20.7  11.4  74.7  88.2                               A60   600        77.5    18.7   7.2  82.6  91.2                               ______________________________________                                    

From the above, about 160A is the maximum thickness for about 70% Tv.500A gave 11.4% Tv which is comparable to that typically found in thedarkest portion of dyed gradient bands (12% in Table 3).

EXAMPLE 3

Though a highly reflective metal gradient band is useful for solarrejection, depending on metal layer thickness, reflective glare into theeyes of occupant(s) within the vehicle passenger compartment can occur.This Example defines a structure for suppressing luminous reflectionwithin a vehicle while maintaining the optical performance improvementsachieved with a reflective metal gradient band described in Example 1.

The approach is to introduce an additional metallic light-absorbinglayer of graduated thickness between highly reflective gradient metalband regions and the interior of the vehicle. Tungsten metal depositeddirectly on a silver layer was used as the absorbing layer. Thus aprepared laminate had the following layer sequence: outboard 5×15 cmglass/tungsten oxide/graduated silver/graduated tungsten tungstenoxide/polyvinyl butyral sheet/inboard 5×15 cm glass. The thicknesses ofthe internal and external tungsten oxide dielectric layers were heldconstant at approximately 450A and 500A respectively across the lengthof the sample. As shown in FIG. 6, silver layer thickness was variedbetween 450A (representative, for example, of that usable with awindshield extension) and 110A (representative, for example, of thatused in the high transmission interference filter component) whiletungsten layer thickness was varied between OA at the start of thegradient silver metal band and 200A at the end of the maximum 450Athickness silver metal band. The optical properties measured at oppositeends of the sample (i.e. at 450 and 110A silver layer thickness) are inTable 5, with the corresponding transmission and reflection spectrashown in FIGS. 7 and 8 respectively, the spectra for the various regionsidentified with the same nomenclature as in FIG. 4. The opticalmeasurements specified for the reflect* region were measured with theincident beam approaching so as to reach the tungsten layer before thesilver layer whereas the reverse was the case for the "clear" and"reflect" regions.

                  TABLE 5                                                         ______________________________________                                        Region  % Tv     % Rv     % As   % Rs   % SR                                  ______________________________________                                        clear   73.4      9.7     26.7   21.8   41.3                                  reflect 5.0      74.0     28.2   68.4   89.0                                  reflect*                                                                              5.0      10.9     73.9   22.7   76.6                                  ______________________________________                                    

From these data it is evident that the tungsten layer is quite effectiveat suppressing luminous reflection (i.e. %Rv=10.9) when the coated glassis viewed from the tungsten rather than the silver side.

To demonstrate more precisely the effect of the anti-glare tungstenbacking layer, several 5×5 cm samples of the following structure wereprepared: outboard glass/tungsten oxide (450A)/silver (500A) tungsten(various thicknesses: 0A, 20A, 50A, 200A)/tungsten oxide (500A)/30 milSaflex sheet/inboard glass. The optical properties in Table 6 weremeasured with the incident beam approaching from both inboard andoutboard sides of a laminate sample.

                  TABLE 6                                                         ______________________________________                                              Tungsten                                                                      thickness                                                               Sample                                                                              (A)        % Tv    % Rv  % As  % Rs  % SR                               ______________________________________                                        A64    0 A       11.4    74.7  20.7  73.1  88.2                                A64*  0 A       11.4    72.7  25.0  68.8  87.1                               A83   20 A       10.5    76.6  20.3  73.9  91.6                                A83* 20 A       10.5    38.5  45.3  48.8  81.9                               A81   50 A        8.7    76.3  21.7  73.2  89.0                                A81* 50 A        8.7    25.7  53.8  41.2  80.5                               A82   200 A       2.6    84.2  21.0  77.3  92.6                                A82* 200 A       2.6    12.4  73.7  24.6  78.4                               ______________________________________                                         *measurement with incident beam approaching from inboard side.           

The above results show that thickening the anti-glare tungsten layerincreases suppression of luminous reflectivity but also decreasesluminous transmission. From these results, when an anti-glare metalbacking component is included in the solar screening film of theinvention, thickness should range between 0 to 300A.

EXAMPLE 4

This shows the effect of grading the thickness of either one or moremetal layers of a gradient band continuation containing plural metallayers.

Two families of samples were prepared. Family #1 had the generalstructure: WO₃ (350A)/Ag(100A)/WO₃ (750A)/Ag(x)/WO₃ (350A) where xvaried from 100 to 600 A. The data for family #1 samples are given inTable 7.

The structure for family #2 was: WO₃ (350A)/Ag(x)/WO₃ (750A)/Ag(x)/WO₃(350A) where x varied from 100 to 500 A. The silver layers were of equalthickness in each sample of family #2. The data for family #2 samplesare given in Table 8.

                  TABLE 7                                                         ______________________________________                                               x                                                                      Smp #  (A)    % Tv     % Rv  % Rs   % As  % SR                                ______________________________________                                        H46    100    78        7    26     24    43                                  H51    200    64       22    43     22    59                                  H50    300    38       47    58     23    74                                  H52    400    29       63    66     19    80                                  H53    500    18       76    72     19    88                                  H54    600     9       83    75     20    89                                  ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                               x                                                                      Smp #  (A)    % Tv     % Rv  % Rs   % As  % SR                                ______________________________________                                        H46    100    78        7    26     24    43                                  H59    200    62       15    47     26    65                                  H62    250    48       29    55     24    73                                  H60    300    37       41    60     23    77                                  H61    400    15       64    69     23    86                                  H63    500     7       75    72     24    90                                  ______________________________________                                    

As evident from Tables 7 and 8, thickening one or both silver layersleads to desirable effects including:

(1) increased solar rejection provided primarily by solar reflection(%Rs) which is considered the most effective rejection mechanism; and

(2) decreased visible transmission (%Tv) so a driver or occupant is lesssubject to glare.

However, though thickening one or both silver layers provide the aboveadvantages, the two approaches are not equivalent and thickening onlyone layer is preferred for the following reasons:

(1) By thickening only one silver layer (Table 7), reflected coloracross the gradient band became either gradually more gold or graduallymore silver depending on which side the sample was viewed from. Thevisually perceived color (i.e. gold or silver) was reflectively uniformshowing change as the silver layer was thickened. On the other hand whenthe thickness of both silver layers was varied (Table 8) the reflectedcolor was first red and then at greater silver thicknesses became goldor silver colored. This variation in color observed across the gradientband for family #2 coatings is probably undesirable.

(2) Although all samples were made with the same sputter coater andconsequently had the same degree of thickness nonuniformity, those withboth silver layers thickened appeared much less uniformly colored inreflection and transmission. This strong sensitivity to unavoidablethickness nonuniformity exhibited by family #2 samples is because of therapid change in reflection intensity (or transmission) over relativelynarrow wavelength bands. The sensitivity of family #2 samples to filmthickness would make these coatings more difficult to produce,particularly on large areas.

The foregoing Examples 1 and 2 illustrate advantages realized using agradient silver layer in a windshield rather than a dyed PVB layer.These advantages are as follows:

(1) Solar rejection for a given visible transmission with a metalgradient is higher than with a dyed gradient. For example both the dyedgradient and the metal gradient in Exs. 1 and 1A (Tables 2 and 3) hadluminous transmission (Tv) of about 12% in the darkest region of thegradient band; however, the metal gradient sample gave 87% SR versus 40%SR with the dyed gradient. Thus the metal gradient would result in asignificantly reduced solar heat load permitting use of a smaller airconditioning unit in a vehicle.

(2) The metal gradient band should provide much better temperaturecontrol during soaking than a dyed gradient which will becomeincreasingly important as front and rear windows in automobiles advanceinto the roof area. It is likely that a highly reflective sectionextending from the top of the high transmission interference filtersegment of a windshield across the top of the vehicle to the clearportion of the rear window will result in lower soaking temperaturesthan will result with a solid painted roof. For example, solarreflectance of Krylon gloss white and gloss black paint was determinedto equal 44% and 4%, respectively. The reflective region of a metalgradient has substantially higher solar reflectance than even whitepaint (e.g. 74% vs. 44%), suggesting that the metal gradient layer wouldbe more effective at suppressing temperature rise in parked closedautos.

While certain specific embodiments of the invention have been describedwith particularity, various modifications will occur to those skilled inthe art. The scope of the invention, therefore, is limited solely by thescope of the following claims.

I claim:
 1. A solar screening film for a motor vehicle windshieldcomprising:(a) an interference filter segment comprising two or morelight-reflecting metal layers, each surface of which is contiguous witha dielectric layer; and (b) an integral gradient band continuation ofsaid interference filter above the direct line of vision through awindshield when said solar screening film is disposed in a motorvehicle, for reflective rejection of infra-red solar energy, said bandincluding an extension of one of the light-reflecting metal layersgradually increasing in thickness toward a marginal edge of the film. 2.The film of claim 1 wherein the two or more light-reflecting metallayers of the interference filter segment are of substantially uniformthickness.
 3. The film of claim 2 wherein the gradient band continuationincludes extensions of each of the dielectric layers of the interferencefilter segment.
 4. The film of claim 3 wherein the thicknesses of theextensions of the dielectric layers of the gradient band continuationare substantially the same as the thicknesses of the dielectric layersof the interference filter segment.
 5. The film of any of claims 1, 2, 3or 4 wherein the varying thickness of the metal layer of the gradientband continuation increases from about 60 to 200 Angstroms at theconfluence with the interference filter segment to about 200 to 1000Angstroms.
 6. The film of claim 5 wherein each metal layer of theinterference filter segment and of the gradient band continuationcomprises silver.